Cathode ray device



June 28, 1938. D A R 2,122,095

CATHODE RAY DEVICE Filed July 13, 1937 2 Sheets-Sheet 1 INVENTOR I QMJune 28, 1938. D. GABoR CATHODE RAY DEVICE Filed July 15, 1957 2Sheets-Sheet 2 INV ENTOR W wma ATTORN EYS Patented June 28, 1938 UNSTATES Dnes Gaibor,

28 Claims.

ates" to devices such as cathscxllogra'phsand cathode ray tubes for 1television, in which oscillograms or electrically ures"ar 'e recorded bya beam of fl scent screen or other re- 'Ifhe" object of this inventionis vic'e fby means of which a once ic'ture' can be made perma- 1 it iswiped out intentioncent screen, in such a way opened remains permabe',called the first, or the beam for which 6. s all be'called the second,device according to the I y ans for producing two with difierent'velocities, and a covered with insulating ls, which are in themondaryelectrons or of good secondary I reas of the secondurface can bemaintained cans hereinafter dee considered as chargeb aims. are directedtocans are provided to evel, that initially, M or faster beam the'slower beam are rethe, invention those mber which are hit am, are madeby this second 'lectric state with state the eleca ejadmitted to thernifof the invention member of the kind fluorescent screen, he" saidmember is a element bfz rsaid i m" V 1 .iaousimiaetweerrxtwo :distincelectrical states, and i sad' l atiitham es *of tri ing-overfrom} thlower .PA'TENT OFFICE CATHODE an nsvrcr.

Rugby, England Application July 13, 1937, Serial No. 153,412

In Great Britain June 4, 1936 state into the upper state involves theemission of secondary electrons.

The novel features which I desire to protect herein will be pointed outin the appended claims. The invention may be best understood by refer- 5once to the following description and the accompanying drawings. In thedrawings Fig. 1 shows a device for demonstrating the principle of theinvention. Fig. 2 is a diagrammatical illustration of the processesinvolved. Fig. 3'shows one form of the cathode ray tube according to theinvention, in which the electrostatic controlling member is itself afluorescent screen. Fig. 4 shows a device by means of which a furtherapplication of the principle of the invention may 16 be demonstrated.Fig. 5 is a diagrammatical illustration of the action of the perforatedcontrolling member or gate utilized therein. Fig.

6 shows a second form of a cathode ray tube according to the invention,with separate controlling member or gate and fluorescent screen. Figs. 7and 8 illustrate the application of cathode ray tubes according to theinvention to television reception. Fig. 9 is a schematic representationof an electron-optical system to produce the second or illuminating beamin television tubes, and Fig. 10 shows a system, in which the means forproducing the two electron beams are united in one construction.

In Fig. 1, which is a section of a device for demonstrating andexplaining the principle of the invention, I is an evacuated envelope, 2and 3 are two indirectly heated cathodes, for producing the first andthe second electron beam respectively. 4 and 5 are heating filaments, 6and 1 accelerating electrodes, which may have a common potential. 8 isa. target on which both electron beams impinge, which may be thinlypowdered with fluorescent substances, for visual demonstration of theeffects in question. 9 is a collecting electrode or anode. Theelectrodes 2, 3, 6-1 and 9 are connected with successively increasingpotentials, whereas the target 8 is connected with the same potential asthe cathode 3, through a high resistance I0. Switches ll and I2 areprovided for connecting the cathodes to the battery and switching in theelectron beams.

If the cathode 3 is switched in by itself, no current will flow to thetarget and no fluorescenoe appears on it, as its potential is the sameas that of the cathode 3, and it repels all electrons. If however thecathode 2 is also switched in, provided that the potentials are suitablychosen, fluorescence appears on the taras get, which persists aftertheswitch II is opened again. v

This phenomenon can be best understood by reference to Fig. 2. In thisdiagram the voltage between the target and the cathode is called E andthe current to the target I. A positive sign means electrons flowing tothe target. In an equilibrium state the current flowing to the targetmust be equal to that leaking away through the resistance R, i. e. itmust be E-/R. This is represented in the diagram by a straight line. Thediagram contains further the characteristics which are obtained if oneor the other of the cathodes is switched in, and the targetv ismaintained at a potential Ea. I1, shown as a dotted line represents theelectron current flowing to the target if only the flrst cathode, (2) isswitched in, the continuous line 1: corresponds to the current from thesecond cathode, (3) Both curves have an s-shape, due to the secondaryelectron emission of the target. which sets in at a certain electronenergy. The current becomes negative if more than one secondary electronis released for every impinging electron. Both curves reach zero againat or near the point Ea, which represents the potential of thecollecting electrode or anode 9.

With only the second cathode working, equilibrium is possible only inthe points 0, A and B, in which the characteristic I: and the resistanceline intersect. Of these three points A is unstable, whereas 0 and B arestable. If the oathode 3 is switched in by itself, the target will restat the point 0. at which it repels all electrons. If however the cathode2 is switched in, the electrons emitted by it will impinge on the targetwith an energy corresponding to the voltage difference Eu between thetwo cathodes. This energy is sufllcient to release more than onesecondary electron per impinging electron, that is, at a ratio greaterthan unity. The potential of the target will therefore rise until itreaches the point B at which equilibrium is again established. Thispoint is beyond the unstable point A. If therefore the beam emitted bythe first cathode is switched ofl again, the second beam will shift theequilibrium point to B. In this condition thesystem is maintained forany time, until the second beam is switched off, and the leakage takesthe system back to 0.

Fig. 3 shows a longitudinal section of a cathode ray tube according tothe invention, in which the above described arrangements and phenomenaare utilized for obtaining on a fluorescent screen permanent records ofpassing electric signals. I3 is an evacuated envelope containing meansfor producing two electron beams of difl'erent velocities. One of these,the flrst and faster beam, is focussed on the fluorescent screen whichis scanned by it in the usual way. This beam is produced by the cathodell, modulated by the electrode i6, accelerated and focussed on thefluorescent screen in the usual way by the electrodes i6, i1 and It withsuccessively increasing potentials. The last accelerating electrode isis.

connected with the tubular member is, which contains the electrostaticdeflecting electrodes. These are arranged in four pairs, in such a way,that the deflected beam passes through an aperture 20. 0f the deflectorsproducing a deflection normal to the plane of the drawings only theplates II and 22 are visible. Deflection in the plane of the drawings isproduced by the pairs 23-24 and 25-28, which are cross-connected in sucha way as to produce opposed deflections, the

canoes anode 2!, which is shown as a conductive coating of the envelope.This anode, which has the highest potential in the whole system,corresponds to the collecting electrode in Fig. 1. One of its functionsis to collect the secondary electrons emitted by the screen 30. Thisscreen consists of fluorescent substance, capable of secondary electronemission, and is backed with a transparent metallic coating 3|,connected with the cathode 21, or with a suitable potential near that ofthe cathode.

It has been found that a fluorescent screen of the kind described,provided that the coating is not too thin, can be considered as built upof a very great number of elements, each of which can exhibitindependently the efl'ects described in connection with Fig. 1. Theresistance It is here replaced by the conductivity of the fluorescentcoating between the surface exposed to the electrons and the metallicbacking. It has been found that most of the usual fluorescent subs anceshave sufiicientiy good secondary electron emission and sumciently low.conductivity to enable small area-elements, with dimensions comparableto the thickness of the coating, to change their equilibrium statesindependently of the surrounding parts by the eflects as abovedescribed. Screens with not sumciently low conductivity may be improvedby applying a semiconductor of sufliciently low conductivity on themetallic coating and applying the fluorescent substance on top.

of this in single grains, which may have also an admixture of goodsecondary emitters.

In the device described, an oscillogram once described by the first beamis made permanently visible by the second beam which spreads out overthe whole area of the screen, hits it however only on those parts wherethe flrst beam has passed. This device is therefore very suitable as anoscillograph for transient electric phenomena, which can be observed orphotographed on the screen for any length of time.

The main application of the invention is however in the fleld oftelevision. Cathode ray tubes for television reception could nothitherto be operated with the very great brightness as required forprojection, for two reasons: The flrst is the destruction of thefluorescent screen at great inputs, the second is the difliculty ofconcentrating very great inputs in small writing spots. The flrstdifliculty is very strongly reduced with the new cathode ray tube, forthe following reasons: The destruction of the screen in cathode raytubes is caused mainly by that the spot covers only about l/100,000 ofthe screen area at a time and rests on it only for about 1/100,000 ofthe time. The whole input, which has to cause a visual impressionlasting at least for 1/50th of a second is transferred to apoint of thescreen in less than a. millionth of a second. In consequence the screenis heated up for a very short time to very high peak temperatures.Artiflcial cooling is of no avail for substantially reducing thesepeaks, as the speed of heat conduc- 16 input densities.

tion is far ,too low for the extraordinary peak According to the presentinvention however the input may be averaged out over a considerablefraction of the total time. If e. g. the bombardment time by theilluminating beam is of the total time, this means that in order tocreate the same screen brightness and the same visual impression theinput density can be reduced to 1/10,000 of its former value. Althoughthe total input remains the same, not only the maximum but also theaverage temperature of the screen is now strongly reduced, as the rateof heat leakage can now easily be made comparable with the rate ofenergy inflow.

The second difficulty, viz. that of concentrating very strong beamcurrents in small writing spots, is also overcome in the tubes accordingto the invention, as the illuminating beam is not concentrated butdiifuse, and covers a large part of the screen area.

A problem particular to television tubes, as distinct from cathode rayoscillographs is that of the modulation of screen brightness. As thetrip-over process on which the invention is based, is a discontinuousprocess, it might seem that the usual methods of intensity modulationcan not be applied. This applies however only to screens which areentirely homogeneous. In reality the unavoidable inhomogeneities of thescreen, due to diflerent grain size, etc. create already a certaindegree of gradation. Grains of different size, of different conductivityand of different secondary electron emissivity can be made to trip overin a certain sequence, in such a way that at a small intensity of thescanning beam only a few trip over into the upper,-permanentlybright,stage, whereas at a certain higher current all trip over andsaturation is reached. This effect may be utilized by mixing thefluorescent substance of grains of different properties. In order toobtain good gradation and the impression of continuously changing screenbrightnesses, it is however also necessary that the spot shall cover atthe same time a great number of grains of different properties.

A second method of effecting modulation according to the invention isthat of building up the picture of points with maximum brightness but ofdifierent size, similarly as in half-tone printing. This may be effectedby modulating not thebeam intensities but the beam diameter. It ismoreover preferable to move the spot in the line direction notcontinuously but in jerks. This may be effected by superimposing on thelinesweep movement a high frequency movement, with a. frequencycorresponding to the desired number of picture points per second. Thissuperimposed oscillation has preferably a saw-tooth waveform, but it hasbeen found that a superimposed sine-wave is suflicient for resolving theline apparently into a series of sharp dots. This method of modulationis also effective only in case of independent screen-elements muchsmaller than the maximum area covered by the scanning spot.

The most effective method consists in a combination of the two methodsabove described, i. e. in a modulation both of spot size and beamintensity and the use of an inhomogeneous screen as above described.

The cathode ray tube according to the invention can be adapted to evenhigher screen inputs by .a modification, the principle of which can beexplained with reference to Fig. 4. This device contains the sameelements as the one shown in Fig. 1, but with the following difference:The electrode 32, corresponding to 8 in Fig. 1,

is made perforated, and behind it is placed a further electrode 33, witha potential higher than that of the collecting electrode 34. If theelectrode 32 is in the state corresponding to "O" in Fig. 2, the currentto the anode 33 due to the broad beam alone will be very small, and itcan be made exactly zero by connecting 32 not to the cathode but to asomewhat lower potential. In the upper state however,-corresponding to Bin Fig. 2,-the electrons of the second broad beam will shoot freelythrough the holes of 32. A part of them impinges on the perforatedelectrode and releases a sufllcient number of secondary electrons formaintaining the high potential.

This effect is utilized according to the invention as shown in Fig. 5.The grid 32 is replaced by a perforated or gauze-like structure 35, withan insulating or semiconducting coating 36. Such a structure may becalled for shortness sake a "gate". It is suiflcient, or even preferableif the coating extends only over the face of the structure exposed tothe electron beams, the other face being left bare. By the insulationthe elements of the surface are made independent of each other. Everyhole may be considered as an independent element, which may or'may nottransmit electrons, independently of the state of the surrounding holes.In Fig. 5 the top opening 3'! is assumed to be at or below cathodepotential, and the electrons of the illuminating, or broad,beam,starting horizontally and shown in continuous lines,are repelled.The scanning beam,shown in dotted lines,just passes the hole 38 in itsupward sweep, and releases secondary electrons which are moving towardsthe collecting electrode. (Secondary electrons are indicated by dot-dashlines.) By this the potential is raised and the passage is opened forthe illuminating beam. In the bottom opening, which has been passed bythe fast beam, the illuminating beam maintains this higher potential, byreleasing secondary electrons.

As rather small voltage differences are sufficient for controlling theilluminating beam current from zero to a maximum, it is advantageous toprovide good secondary emitters on the surface of the insulating orsemiconducting coating, i. e. substances from which electrons withrather low energy may release more than one electron. By this thevoltage jump between the lower and the upper state is reduced. A highvoltage jump represents in this case a waste, whereas in the deviceshown in Mg. 3 this voltage difference is useful, as it serves forexciting the fluorescent screen.

Fig. 6 shows a device according to the invention utilizing a gate". Themeans for producing the two electron beams are the same as described inconnection with Fig. 3. In this device however no fluorescent screen buta gauze 40, prepared in the above described manner, is scanned by thefast beam. This gauze is held by the frame 4|. It is electron-opticallyimaged on the fluorescent screen 42. The electron-optical system iscomposed of the electrodes 43 and 44, shown as coatings of theenvelope,and the magnet coil 45.

As compared with the device shown in Fig. 3, this form of the inventionhas the advantage, that the accelerating voltage between 40 and 42 canbe chosen freely, independently of the secondary emitting properties ofthe gate". It is therefore possible to realize by means of this deviceeven higher screen inputs than with the first form. This device istherefore especially suited for television projection on large screens,e. g. cinema theatre screens.

In the reproduction of television pictures the difilculty arises, thatif the illuminating beam is switched oif once after every frame orpicture, those parts of the picture which have been traced last willemit less light than those traced at the beginning. This difficulty isovercome according to the invention with the arrangementdiagrammatically illustrated in Fig. 7. In this figure 66 is the controlscreen such as the screen of Fig. 3 or gate 60 of Fig. 6, as the casemay be, on which the potential pattern is produced. 61 represents theelectron gun for producing the scanning beam. 48-46 and 606l are the twopairs of deflecting plates producing the scanning. 62 representsdiagrammatically the large cathode emitting the second broad beam, whichirradiates the screen on a band 55 and is deflected by the plates 53,56. The deflections of the two beams are matched in such a way, that theilluminating beam follows the scanning beam, its upper edge moving alittle ahead of it. The picture lines are indicated by dotted lines. Ifin this arrangement the width of the illuminating band is chosen equalto or a little less than the dark spaces following each frame, everypicture point will receive equal illumination. By this method it istherefore possible to utilize about 10% of the time, as compared withabout .001 by the usual method.

A further improvement is effected by the method diagrammaticallyillustrated in Fig. 8. This method allows utilizing almost the wholetime for electron bombardment. In this figure only the screen,or gate,66is shown, with means for producing and deflecting the scanning beam,which are the same as in the previous figure. The illuminating beamcovers here the whole area of the screen, with the exception of a narrowdark band 56, which moves a little ahead of the scanning spot 51. Thisdark band wipes out the picture, by giving the elements of the screen orgate just sufiicient time to discharge. The previous picture 66 changestherefore almost continuously into the next picture 60, and the flickeris reduced to a minimum.

Fig. 9 is a simplified representation of an elec tron-optical device forproducing full illumination of the screen, interrupted only by a thindark band. A cathode 6i of sufilciently large area is surrounded by aguard ring 62. The electrons emitted by the cathode are accelerated bythe tubular electrode 66 and thrown at the disc 66, which itself has asuitable positive potential. This disc is fitted with two windows 66,66, each of which has the proportions of the picture, and which aredivided by a narrow cross-bar 61. The electrostatic lens system 66, 69is adjusted in such a way that a strongly magnified sharp image of thewindow with the cross-bar appears on the screen. The electrode 63, orother condensing system by which it may suitably be replaced, isarranged in such a way as to produce uniform illumination of the windowand therefore of the picture. The whole image of 66 and 66 is movedduring the course of the frame scan across the screen by means of thedeflecting electrodes 10 and II. On the screenthere will appeartherefore a dark band which wipes out the preceding picture before thenext one takes its place. In order to avoid wiping out the pictureduring the fiy-back of the dark band, the followingv method may be usedaccording to the invention: During the fiy-back the potential of one orboth of the focussing electrodes 66. 66 is changed in such a way thatthe picture of the window is defocumed and the image of the cross-barvanishes almost completely.- If however the fiy-back is sufilcientlyrapid, and the time of the passage of the dark strip is considerablyshorter than the time constant of discharge of the screen or gateelements, this defocussing is unnecessary.

Fig. 10 shows a section of a gun construction, containing the means forproducing and defiecting both electron beams, in symmetricalarrangement. This consists of the cathode I! for the first or scanningbeam, with the focussing system I6, held by the insulating part I6 inthe tubular member 16, which carries the deflector system for thescanning beam. v'Zll'hese are arranged in four pairs, corresponding toFigs. 3 or 6, arranged in such a way as to make the deflected beam pivotaround the centre of the aperture 16.

The organs for producing the second or illuminating beam are essentiallythe same as in Fig. 9, and are arranged near the end of the tubularmember 16. The cathode consists of an annular ring 11 of triangularcross section, enclosing the heating filament I6. Its active surface hasthe shape of a truncated cone, situated behind the diaphragm IS. Thecondensing electrode 66 is arranged in such a way, that the cathode raysemitted by 11 illuminate the window 16 uniformly. This window has thesame shape as described in connection with Fig. 9 and has a thin crossbar 6i in its middle. It is followed by the focussing electrodes 6! and66 and by the deflecting electrodes 66 and 66.

As the potential of the cathode I2 is considerably lower than that of11, the electrons of the scanning beam have a much higher velocity thanthose of the illuminating beam. They will suffer therefore only a smalladditional focussing and deflecting eflect by the electrodes 66-46. Bysuitable dimensioning of the gun system and of the applied voltages itis possible to obtain at the screen simultaneously a sharp spot for thescanning beam and a sharp image of the cross bar.

While I have shown particular embodiments of my invention, it will beunderstood that many modifications and applications may be made by thoseskilled in the art, without departing from the invention as set forth inthis specification and in the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a cathode ray signal reproducing tube, the combination whichcomprises a control screen having a conductive base and asecondary-electron-emissive charge-retaining surface thereon connectedto the conductive base through resistive material, an electron gun forproducing a scanning beam of electrons of relatively high velocitypositioned and adapted to impinge on said control screen, deflectingmeans for said scanning beam, a second electron gun for producing abroad beam of electrons of relatively low velocity positioned andadapted to impinge on said control screen, said second electron gunhaving an electrode including a window having a cross bar thereacrossadapted to divide said broad beam of electrons into two portionsseparated by a band free of electrons, means for electron-opticallyimaging said window on said control screen, a collecting anodepositioned to collect secondary electrons emitted from said controlscreen, the

conductive base of the control screen being connected to the cathode ofthe broad beam electron gun so that initially, prior to the passing ofthe scanning beam, electrons of the broad beam will not liberatesecondary electrons from the emissive surface thereof at a ratio greaterthan unity, the relatively high velocity electrons of the scanning beambeing adapted to liberate secondary electrons from the scanned areas ofthe control screen at a ratio greater than unity, said secondaryelectrons being collected by said collecting anode and thereby raisingthe potentials of the scanned areas so that electrons of the broad beamthereafter impinge on the scanned areas at increased velocities andliberate secondary electrons at a ratio greater than unity to maintainthe scanned areas at elevated potentials so long as electrons in thebroad beam continue to impinge thereon, thereby creating a potentialpattern on said control screen, means for utilizing said potentialpattern to reproduce an image corresponding thereto, and deflectingmeans for deflecting across the control screen said portions of thebroad beam separated by said band free of electrons and thereby permitthe charge on the scanned emissive areas lying at said band to leak awaythrough the resistive material and restore the areas to their initialpotentials.

2. In a cathode ray signal reproducing tube, the combination whichcomprises a control screen having a conductive base and asecondary-electron-emissive charge-retaining surface thereon connectedto the conductive base through resistive material, an electron gun forproducing a scanning beam of electrons of relatively high velocitypositioned and adapted to impinge on said control screen, deflectingmeans for said scanning beam, a second electron gun for producing abroad beam of relatively low velocity electrons positioned and adaptedto impinge on said control screen in a band covering a plurality ofscanning lines, a collecting anode positioned to collect secondaryelectrons emitted from said control screen, the conductive base of thecontrol screen being connected to the cathode of the broad beam electrongun so that initially, prior to the passing of the scanningbeam,'electrons of the broad beam will not liberate secondary electronsfrom the emissive surface thereof at a ratio greater than unity, therelatively high velocity electrons of the scanning beam being adapted toliberate secondary electrons from the scanned areas of the controlscreen at a ratio greater than unity, said secondary electrons beingcollected by said collecting anode and thereby raising the potentials ofthe scanned areas so that electrons of the broad beam thereafter impingeon the scanned areas at increased velocities and liberate secondaryelectrons at a ratio greater than'unity to maintain the scanned areas atelevated potentials so long as electrons in the broad beam continue toimpinge thereon, thereby creating a potential pattern on said controlscreen, means for utilizing said potential pattern to reproduce an imagecorresponding thereto, and deflecting means for deflecting the broadbeam across the control screen and thereby permitting the charges on thescanned areas from which the broad beam has been deflected to leak awaythrough the resistive material and restore the areas to their initialpotentials.

3. In a cathode ray signal reproducing tube. the combination whichcomprises a perforated conductive control grid having on the surfacethereof secondary-electron-emissive charge-retainingeiements k H vhrousresistive mafiefla eh l ctrqasun or: P du i a scanning be'ai'n o'felectrons o relativelyqhigh velocity positioned 3 and adapt to impinge,con said control'grid,deflecting me ns ,said scan- 5 ning beam, asecond electrongun;;-for, producing a broad beam ofelectrons of,;relatively low1velocity positioned and adapted to impinge on .saidcontrol grid'infa band covering a pluralitysofm scanning lines,acollecting anode positioned; and 10 adapted to collect secondaryelectronmemitted from said control grid, a luminescent screenpositionedto receive electrons or -said broad-beam passing through" said a,control, grid and adapted; to be rendered luminescent thereby the. con-15 ductive control grid being connected to the, cathode of the, broadbeam electron'gunsothatinitial- 1y, prior to the passingoftheiscanningbeam the potentialsof the emissive elements ;thereof wi1l1 substantially prevent the. electrons, of the, broad beam fromimpinging on said luminescent screen and electrons in the broad,beam,wil1 ;not-.;li b erate secondary electrons fro t ments at a ratio'greater than; ly high velocity electronsfof t being adaptedto'fliber'ate from, the scanned areas at a unity, said secondary electroby said collecti g anodefand potentials of the scanned are of the broad;beam tn 'raftej scanned areas at increased "velo secondary 'electroiisfata ratio to maintain the scannedeleme tentials so long as electronsi continue to impinge "there tentials permitting electrons to passthrough corresponding'are trol grid and impinge on the lumin anddefiectingf meansf'for' fsaid deflecting the beam away fr and therebypermitting the' ha sive elements to lealraway thr material and restoringthe initial potentialsi 4. In a cathode-ray *signal conductive control;grid? h'avin thereof secondary retaining element A p H throughresistive' *rnatferi producing a scanning n atively high velocitypo'sition impinge 'onisam centre for said 'scanning' bea for producing abroad atively low velocity po dow having =a'cros's bar dividesaid broadbe means for electron-optically imagi'n dow at saidwcontrol grid;.a-*colleetin' ridcanda-adaptedst erebmthe: Conductive m1 h l I a on saident creen and electrons in the broad beam will not liberate secondaryelectrons from the emissive elements at a ratio greater than unity, thehigher velocity electrons of the scanning beam being adapted to liberatesecondary electrons from the scanned areas at a ratio greater than unitywhich are collected by the said collecting anode, the potential of thecollecting anode being sufficiently high so that the potentials of thescanned areas are raised by said secondary electron emission so thatelectrons of the broad beam thereafter impinge on the scanned areas atincreased velocities and liberate secondary electrons at a ratio greaterthan unity to maintain the scanned elements at elevated potentials solong as electrons in the broad beam continue to impinge thereon, saidelevated potentials permitting electrons in the broad beam to passthrough corresponding areas of the control grid and impinge on theluminescent screen, and deflecting means for deflecting across thecontrol grid said portions of the broad beam separated by said band freeof electrons and thereby permit the charge on the scanned emissiveelements lying at said band to leak away through the resistive materialand restore the elements to their initial potentials.

5. In a cathode-ray signal reproducing tube,

the combination which comprises a perforated conductive control gridhaving on the surface thereof secondary-electron-emissivecharge-retaining elements connected to said grid through resistivematerial, an electron gun for producing a scanning beam of electrons ofrelatively high velocity positioned and adapted to impinge on saidcontrol grid, deflecting means for said scanning beam, a second electrongun for producing a broad beam of electrons of relatively low velocitypositioned and adapted to impinge on said control grid in a bandcovering a plurality of scanning lines, a collecting anode positioned tocollect secondary electrons emitted from said control grid, aluminescent screen positioned to receive electrons of said broad beampassing through said control grid and adapted to be rendered luminescentthereby, the conductive control grid being connected to the cathode ofthe broad beam electron gun so that initially, prior to the passing ofthe scanning beam, the potentials of the emissive elements thereof willsubstantially prevent the electrons of the broad beam from impinging onsaid luminescent screen and electrons in the broad beam will notliberate secondary electrons from the emissive elements at a ratiogreater than unity, the higher velocity electrons of the scanning beambeing adapted to liberate secondary electrons from the scanned areas ata ratio greater than unity which are collected by the said collectinganode, the potential of the collecting anode being sufilciently high sothat the potentials of the scanned areas are raised by said secondaryelectron emission so that electrons of the broad beam thereafter impingeon the scanned areas at increased velocities and liberate secondaryelectrons at a ratio greater than unity to maintain the scanned elementsat elevated potentials so long as electrons in the broad beam continueto impinge thereon, said elevated potentials permitting electrons in thebroad beam to pass through corresponding areas of the control grid andimpinge on the luminescent screen, and deflecting means for said broadbeam for deflecting the beam away from the scanned areas and therebypermitting the charge on said emissive elements to leak away through theresistance material and restoring the elements to their initialpotentials.

6. A cathode ray signal reproducer tube which comprises an electron gunfor producing a scanning beam of relatively high velocity electrons, asecond electron gun for producing a broad beam of relatively lowvelocity electrons, a secondary-electron-emissive charge-retaining lumiinescent screen positioned and adapted to be impinged by said broad beamand said scanning beam of electrons, and a collecting anode positionedand adapted to collect secondary electrons emitted from said screen, thevelocity of the electrons in said broad beam being initially too low tostrike said luminescent screen and render it substantially luminescentand too low to liberate secondary electrons therefrom at a ratio greaterthan unity, the electrons in said scanning beam being of sufllcientlyhigh velocity to liberate secondary electrons from the scanned areas ata ratio greater than unity, said secondary electrons being collected bysaid collecting anode at a sumciently high potential to thereby raisethe potentials of the scanned areas so that electrons of the broad beamthereafter impinge on the scanned areas at increased velocities torender said scanned areas luminescent and to liberate secondaryelectrons therefrom at a ratio greater than unity to maintain thescanned areas at increased potentials and thereby maintain saidluminescence so long as electrons in the broad beam continue to impingethereon.

7. A cathode ray signal reproducer tube which comprises an electron gunfor producing a scanning beam of relatively high velocity electrons,deflecting means for said scanning beam, a reproducing screen having aconductive base and secondary electron emissive charge retainingelements on the surface thereof connected to said conductive basethrough resistive material, said reproducing screen being adapted to berendered luminescent by electrons impinging thereon, a second electrongun for producing a broad beam of relatively low velocity electronspositioned and adapted to impinge on said reproducing screen in a bandcovering a plurality of scanning lines produced by said scanning beam, acollecting anode positioned and adapted to collect secondary electronsemitted from said screen, the conductive base of said reproducing screenbeing maintained at a potential such that the electrons in saidbroadbeam have velocities initially too low to strike said reproducing screenand render it substantially luminescent and too low toliberate secondaryelectrons therefrom at a ratio greater than unity, whereas the electronsin said scanning beam have suillciently high velocities to liberatesecondary electrons from the scanned areas at a ratio greater thanunity, said collecting anode being maintained at a sufllciently highpotential to collect said secondary electrons to raise the potentials ofthe scanned areas so that electrons of the broad beam thereafter impingeon the scanned areas at increased velocities to render said scannedareas luminescent and-to liberate secondary electrons therefrom at aratio greater than unity to maintain the scanned areas at increasedpotentials, thereby maintaining said luminescence so long as electronsof the broad beam continue to impinge on the scanned areas, anddeflecting means for said broad beam for deflecting the beam away fromthe scanned areas and thereby permitting the charges on the scannedelements to leak away through the rel sistive material to the conductivebase and restore the elements to their initial potentials.

8. In cathode-ray tube signal reproducing apparatus in which thereproduction persists after the signal has passed, the combination whichcomprises means for producing a concentrated electron scanning beam anda broad electron beam of different velocities, a secondary-electronemissive surface positioned to be impinged by said scanning and saidbroad electron beams, means for impressing upon said emissive surface aninitial potential such that prior to the passing of said scanning beamover elemental areas thereof the electrons insaid broad beam do notraise the potentials of said elemental areas, whereas electrons in saidscanning beam do raise the potentials of the scanned elemental areas bysecondary electron emission therefrom to an extent suflicient to causeelectrons in said broad beam to thereafter impinge on the scanned areasat velocities suflicient. to maintain the scanned areas at elevatedpotentials by secondary electron emission therefrom so long as electronsin the broad beam continue to impinge thereon.

9. In cathode-ray tube signal reproducing apparatus in which thereproduction persists after the signal has passed, the combination whichcomprises means for producing a concentrated electron scanning beam anda broad electron beam of different velocities, a secondary-electronemissive surface positioned to be impinged by said scanning and saidbroad electron beams, elemental areas of said surface beingcharge-retaining, means for impressing upon the elemental areas of saidemissive surface initial potentials such that prior to the passing ofthe scanning beam thereover the electrons in said broad beam do notliberate secondary electrons therefrom at a ratio greater than unity,whereas electrons in said scanning beam do liberate secondary electronsat a ratio greater than unity and thereby raise the potentials of thescanned areas an extent suflicient to cause electrons in said broad beamto thereafter impinge on the scanned areas at velocities sufficient tomaintain the scanned areas at elevated potentials by secondary electronemission at a ratio greater than unity so long as'electrons in the broadbeam continue to impingethereon.

10. In cathode-ray tube signal reproducing apparatus in which thereproduction persists after the signal has passed, the combination whichcomprises a secondary-electron emissive screen, elemental areas of whichare charge-retaining; means for producing a potential pattern on saidscreen corresponding to the signal to be recorded comprising means forproducing a scanning beam of relatively high velocity electrons and abroad beam of relatively low velocity electrons, said scanning beambeing adapted to impinge on said screen in an elemental area and saidbroad electron beam being adapted to impinge on said screen over arelatively large area, means for impressing upon elemental areas of theemissive screen initial potentials such that prior to. the passing ofthe scanning beam thereover the electrons in said broad beam do notliberate secondary.electrons therefrom at a ratio greater than unitywhereas electrons in said scanning beam do liberate secondary electronsat a ratio greater than unity and thereby raise the potentials of thescanned areas an extent sufiicient to cause electrons in said broad beamthereafter to liberatesecondary electrons from the scanned areas at aratio greater than unity and maintain thescanned areas at elevatedpotentials so long as electrons in the broad beam continue to impingethereon, thereby producing a potential pattern on said screen; and meansfor utilizing the potential pattern so produced to reproduce an imagecorresponding thereto.

11. In. cathode-ray tube reproducing appa-' ratus for producing on aluminescent screen records of electric signals which persist after thesignals have passed, the combination which comprises an electron gunincluding a cathode and an anode co-operating therewith to define anelectron scanning beam of elemental cross-section, a second electron gunincluding a cathode and an anode co-operating therewith to define abroad electron beam of relatively large cross-section, means formaintaining the cathode of the first electron gun at a lower potentialthan the cathode of the second electron gun, a control screen positionedin the paths of said electron beams, said control screen beingconductive and having thereon a layer of substantially discretesecondaryelectron emissive elements connected to the screen throughresistive material, a collecting anode positioned to collect secondaryelectrons emitted from said elements, a luminescent screen positionedand adapted to receive electrons of said broad beam under the control ofsaid control .screen, means for maintaining the conductive screen at apotential with respect to said cathodes such that initially, prior tothe passing of the scanning beam, the electrons in the broad beam willbe substantially prevented from producing an image on said luminescentscreen and the potential difference between the emissive elements andthe cathode of the second electron gun will be less than required forthe emission of secondary electrons from the elements in a ratiosufficient to raise the potential of the elements, whereas the potentialdifference between the emissive elements and the cathode of the firstelectron gun, in conjunction with the potential of the collecting anode,will cause electrons in the scanning beam to liberate secondaryelectrons from the scanned emissive elements in a ratio sufiicient toraise the potential of the scanned elements to at least the potentiallevel at which electrons from the second cathode also release secondaryelectrons in a ratio suflicient to balance the leakage current from thescanned elements in the conducting screen, thereby maintaining thescanned emissive elements at at least said potential level so long aselectrons in the broad beam continue to impinge thereon, means formodulating the scanning beam to create a potential pattern of elementsmaintained at at least said potential level corresponding to the signalto be recorded, said potential level admitting electrons of said broadbeam to said luminescent screen to produce an image thereon.

12. In apparatus for producing an optical image including a recordingscreen adapted to be rendered luminescent by electrons impinging thereonunder the control of a potential pattern created on a secondary electronemissive chargeretaining surface, the method of creating said potentialpattern which comprises initially exposing said secondary-electronemissive chargeretaining surface to a relatively broad uniform stream ofelectrons whose velocities are too low to strike said surface and emitsecondary electrons at a ratio greater than unity, and bombardingelemental areas of said surface by a scanning beam of relatively highvelocity electrons to raise the potentials of said areas by secondaryelectron emission therefrom to an extent suiiicient to cause electronsof said broad beam to thereafter impinge on said areas at increasedvelocities sumcient to maintain said areas at raised potentials so longas the exposure of the scanned areas to said broad beam is continued.

13. In cathode-ray tube apparatus for producing images by a flow ofelectrons under the control of a potential pattern created on asecondaryelectron emissive surface, elemental areas of which arecharge-retaining, the method of creating said potential pattern whichcomprises creating a broad beam of electrons and directing the beamtoward said secondary-electron emissive surface, the velocities of saidbroad beam electrons being initially insufficient to raise thepotentials of said elemental areas, and raising the potentials ofelemental areas of said emissive surface by bombarding said areas, inthe presence of said broad electron beam, with a relatively highvelocity scanning beam of electrons to an extent such that electrons ofthe broad beam will thereafter impinge on the scanned areas at increasedvelocities suflicient to maintain the scanned areas at elevatedpotentials by secondary electron emission therefrom so long as the flowof electrons in the broad beam to the scanned areas is continued,thereby producing a potential pattern on said emissive surface.

.. 14. In cathode-ray tube apparatus for producing images by a flow ofelectrons under the control of a potential pattern created on asecondaryvelectron emissive surface, the method of creating saidpotential pattern which comprises initially exposing saidsecondary-electron emissive surface to a broad beam of electrons havingvelocities too low to strike said emissive surface and liberatesecondary electrons at a ratio suflicient to raise the potentials ofelemental areas of said surface, and scanning elemental areas of saidsurface in the presence of said broad beam with a scanning beam ofelectrons having velocities sufliciently high to liberate secondaryelectrons to raise the Potentials of the scanned areas to such an extentas to cause electrons of the broad beam to thereafter impinge on saidscanned areas at velocities suflicient to liberate secondary electronsto maintain said areas at raised potentials so long as the flow of theelectrons in the broad beam to the scanned areas is continued, therebyproducing a potential pattern on said emissive surface. a

15. The method of producing an image by a cathode-ray tube whichcomprises creating a broad beam of electrons for producing an image,controlling the production of said image by said broad beam by means ofa secondary-electronemissive surface positioned in the path of saidbroad beam, initially maintaining said emissive surface at a potentialsuch that electrons of the broad beam will be substantially preventedfrom producing an image, and scanning elemental areas of said emissivesurface with a scanning beam of relatively high velocity electrons toraise the potentials of the scanned areas by secondary electron emissionto a degree such that electrons of the broad beam will thereafterimpinge thereon with suillciently high velocities to maintain thescanned areas at raised potentials so long as the flow of the electronsin the broad beam to the scanned areas is continued, the raisedpotentials of said scanned areas permitting the electrons of the broadbeam to produce an image corresponding thereto.

16. The method of producing an image by a cathode-ray tube whichcomprises creating a broadbeamofrelativelylowvelocityelectronsforproducing an image on a recording surface, controlling the production ofsaid image by said broad beam by means of a secondary-electrom emissivecharge-retaining surface positioned in the path of said broad beam,initially maintainingsaidemissivesurface atapotentialsuchthat electronsin said broad beam will be substantially prevented from producing animage and will not raise the potential thereof by secondary-electronemission, and scanning elemental areas of said emissive surface in thepresence of said broad beam with a scanning beam of relatively highvelocity electrons to raise the potentials of the scanned areas bysecondary electron emission therefrom to an extent such that electronsof the broad beam will thereafter impinge thereon with sufliciently highvelocities to maintain the scanned areas at raised potentials bysecondaryelectron emission therefrom so long as the flow of electrons inthe broad beam to the scanned areas is continued, the raised potentialsof said scanned areas permitting electrons of the broad beam toproducean image on the recording surface.

17. The method of producing an image on a recording surface by acathode-ray tube which comprises initially exposing to a relativelybroad beam of electrons a screen having secondaryelectron-emissivecharge-retaining elements on the surface thereof, the velocity of saidelectrons being too low to raise the Potentials of said elements by theemission of secondary electrons at a ratio greater than unity, scanningelemental areasof said screen duringsaidexposurebya scanning beam ofrelatively h velocity electrons to emit secondary electrons at a ratiogreater than unity, collecting said secondary electrons at a potentialsuiiiciently high to cause the potentials of the scanned elemental areasto be raised by said secondary electron emission to an extent sufficientto cause electrons in said broad beam to thereafter emit secondaryelectrons at a ratio greater than unity and thus maintain said scannedareas at elevated potentials so long as the flow of electrons in thebroad beam to the scanned areas is continued, thereby producing apotential pattern on said screen, and utilizing said potential patternto control the flow of elecimage by electrons impinging thereon.

18. The method of producing an image on a recording surface by acathode-ray tube which comprises creating a broad beam of relatively lowvelocity electrons for producing an image on a recording surface,controlling the admission of electrons of said broad beam to saidrecording surface by meansof a secondary-electron-emissive screenpositioned in the path of said broad beam, elemental areas of saidscreen being charge-retaining, initially maintaining said elementalareas at such low potentials that electrons in said broad beam will besubstantially prevented fromproducing an image on said recording screenand will not raise the potentials of said areas by secondary electronemission therefrom, scanning elemental areas of said screen in thepresence of said broad beam by a scanning beam of relatively highvelocity electrons to emit secondary electrons at a ratio greater thanunity, collecting said secondary electrons at a potential sufiicientlyhigh to cause the potentials of the scanned elemental areas to be raisedby said secondary electronv emission to an extent sufilcientto causeelectrons in said broad beam to thereafter emit secondary electrons at aratio greater than unity and thus maintain said scanned areas atelevated potentials so long as the flow of electrons in the broad beamto the scanned areas is continued, the raised potentials of said scannedareas permitting electrons of the broad beam to produce an image on therecording screen corresponding thereto.

19. The method of producing an image on a recording surface by acathode-ray tube which comprises creating a broad beam of relatively lowvelocity electrons for producing an image on a recording surface,controlling the admission of electrons of said broad beam to saidrecording surface by means of a secondary-electron-emissive screenpositioned in the path of said broad beam, elemental areas of saidscreen being charge-retaining, initially'maintaining said elementalareas at such low potentials that electrons in said broad beam will besubstantially prevented from producing an image on said recording screenand will not raise the potentials of said areas by secondary electronemission therefrom, scanning elemental areas of said screen in thepresence of said broad beam by a scanning beam of relatively highvelocity electrons to .emit secondary electrons at a ratio greater thanunity,

collecting said secondary electrons at a potential sumciently high tocause the potentials of the scanned elemental areas to be raised by saidsecondary electron emission to an extent sufiicient to cause electronsin said broad beam to thereafter emit secondary electrons at a ratiogreater than unity and thus maintain said scanned areas at elevatedpotentials so long as theflow of electrons in the broad beam to thescanned areas is continued, the raised potentials of said scanned areaspermitting electrons of the broad beam to produce an image on therecording screen corresponding thereto, and periodically restoring theelemental areas of said emissive screen to substantially their initiallow potentials for rescanning thereof.

20. In a cathode-ray tube signal reproducing apparatus for producing,screen, records of electric signals which persist after the signals havepassed, the combination which comprises means for producing a scanningelectron of elemental cross-section and a broad electron beam ofrelatively large crosssection, the electrons in the scanning beam havinghigher velocities than the electrons in the broad beam, asecondary-electron emissive reproducing screen adapted to reproduceimages by the impingement of electrons thereon, said electron beamsbeing directed toward said reproducing screen, means for scanningelemental areas of said reproducing screen with said scanning beam inthe presence of said broad beam, means for maintaining elemental areasof said screen initially, prior to the passing of the scanning beamthereover, at such potentials that electrons in said broad beamsubstantially do not reproduce an image on said areas and do not raisethe potentials thereof by secondaryelectron emission therefrom, whereasat said potentials the higher velocity electrons in the impinge thereonat increased velocities sufilcient on a reproducing to maintain saidareas at at least said potential level by secondary electron emissiontherefrom at a ratio greater than unity so long as electrons in saidbroad beam continue to impinge thereon. said increased velocities of theelectrons in the broad beam being suiiicient to reproduce an image onsaid reproducing screen.

21. In cathode-ray tube signal reproducing apparatus in which thereproduction persists after the signal has passed, means for producing apotential pattern for controlling the reproduction of, an imagecorresponding thereto which comprises, in combination, means forproducing a scanning electron beam of elemental crosssection and a broadelectron beam oi relatively large cross-section, the electrons in thescanning beam having higher velocities than the electrons in the broadbeam, a control screen having a secondary-electron emissive surfacethereon elemental areas of which have substantial resistancetherebetween, means for scanning elemental areas of said emissivesurface with said scanning beam in the presence of said broad beam,means for impressing upon elemental areas of said emissive surfaceinitial potentials such that prior to the passing of the scanning beamthereover the electrons in the broad beam do not raise the potentialsthereof by secondary electron emission therefrom, whereas at saidpotentials the higher velocity electrons in the scanning beam liberatesecondary electrons from the scanned areas at a ratio greater thanunity, and means for collecting. the secondary electrons liberated bythe scanning beam at a potential suficiently high so that the potentialsof the scanned areas are raised to at least the level at which electronsin the broad beam thereafter impinge thereon at increased velocitiessufhcient to maintain said scanned areas at at least said potentiallevel by secondary electron emission therefrom at a ratio greater thanunity so long as electrons in the broad beam continue to impingethereon, whereby a potential pattern may be created on said controlscreen.

22. In a cathode-ray tube signal reproducing apparatus for producing, ona luminescent screen, records of electric signals which persist afterthe signals have passed, the combination which comprises means forproducing a scanning electron beam of elemental cross-section and abroad electron beam of relatively large cross-section, the electrons inthe scanning beam having higher velocities than the electrons in thebroad beam, a secondary-electron emissive surface, said electron beamsbeing directed toward said emissive surface, a luminescent surfaceadapted to be rendered luminescent by electrons impinging thereon andpositioned to be impinged by electrons of said broad beam under thecontrol of said emissive surface, means for maintaining elemental areasof said emissive surface initially, prior to the passing of the scanningbeam thereover, at such low potentials that the slower electrons of thebroad beam do not release secondary electrons therefrom at a ratiogreater than unity and do not produce substantial luminescence of saidluminescent surface, whereas at said potentials the faster electrons ofthe scanning beam release secondary electrons from the scanned areas ata ratio greater than unity and thereby raise the potentials of thescanned areas to a level at which the electrons of the broad beam alsorelease secondary electrons at a ratio greater than unity and suiiicientto maintain said scanned areas at at least said scanning electron beamof elemental cross-section, a second electron gun adapted to produce arelatively broad electron beam, said electron guns having respectivecathodes, means for maintaining the cathode of the first electron gun ata lower potential than the cathode of the second electron gun, a controlscreen positioned in the path of said electron beams having a conductivemember and a secondary-electron emissive surface exposed to saidelectron beams, elemental surface areas of said emissive surface havingsubstantial resistance therebetween and being connected to theconductive member through substantial resistance, a collecting anodepositioned to collect secondary electrons emitted from said emissivesurface, a luminescent screen positioned and adapted to receiveelectrons of said broad beams under the control of said control screen,means for scanning elemental areas of said emissive surface with saidscanning beam in the presence of said broad beam, means for maintainingthe potential of said conductive member at a potential near that of thecathode of the second electron gun such that initially, prior to thepassing of the scanning beam over elemental areas of the emissivesurface, the broad beam electrons are substantially prevented fromproducing an image on said luminescent screen and the potentialdifference between the elemental surface areas and the cathode of thesecond electron gun is less than required for electrons in-the broadbeam to raise the potentials of said areas by secondary-electronemission therefrom at a ratio greater than unity, the potential of thecathode of the first electron gun being selected with respect to thepotential of the conductive member so that electrons in the scanningbeam liberate secondary electrons from scanned areas at a ratio greaterthan unity, the potential of the said collecting anode being maintainedsufllciently high so that the potentials of the scanned areas are raisedby the said liberation of secondary electrons therefrom to at least thepotential level at which electrons in the broad beam also liberatesecondary electrons in a ratio greater than unity and sufficient tobalance the leakage current from the scanned areas to the conductingmember, thereby maintaining the scanned areas at at least said potentiallevel, so long as electrons in the broad beam continue to impingethereon, said potential level admitting electrons of said broad beam tosaid luminescent screen to produce an image thereon, and means forperiodically discontinuing the flow of electrons in the broad beam tothe scanned areas to permit the charge on the scanned areas to leak awayto the conductive member and restore the scanned areas to substantiallytheir initial potentials at which the electrons in the broad beam aresubstantially prevented from producing an image on the luminescentscreen.

24. In cathode-ray tube signal reproducing apparatus for producing, on aluminescent screen,

records of electric signals which persist after the signals have passed,the combination which comprises a control screen having asecondary-electron emissive surface elemental areas of which havesubstantial resistance therebetween, means for producing a scanningelectron beam of elemental cross-section and a broad electron beam ofrelatively large crosssection, the electrons in the scanning beam havinghigher velocities than the electrons in the broad beam, a luminescentsurface adapted to be rendered luminescent by electrons impingingthereon and positioned to be impinged by electrons of said broad beamunder the control of said emissive surface, means for scanning elementalareas of said emissive surface with said scanning beam in a plurality oflines in the presence of said broad beam electrons, said broad beamcovering an area of said emissive surface including a plurality ofscanning lines, means for maintaining elemental areas of said emissivesurface initially, prior to the passing of the scanning beam thereover,at such low potentials that the slower electrons of the broad beam donot release secondary electrons therefrom at a ratio greater than unityand do not produce luminescence of said luminescent surface, whereas atsaid potentials the faster electrons in the scanning beam liberatesecondary electrons from the scanned areas at a ratio greater thanunity, means for collecting the secondary electrons liberated by thescanning beam at a potential sufllciently high so that the potentials ofthe scanned areas are raised to at least the level at which electrons inthe broad beam thereafter impinge thereon at increased velocitiessumcient to maintain said scanned areas at at least said potential levelby secondary electron emission therefrom at a ratio greater than unity,whereby the raised potentials of the scanned areas persist after thescanning beam has passed on so long as electrons in said broad beamcontinue to impinge thereon, said raising of the potentials of thescanned areas admitting electrons in the broad beam to said luminescentsurface to produce a luminescent image thereon, and means correlatedwith the deflection of the scanning beam for deflecting the broad beamin a direction substantially perpendicular to the direction of thescanning lines to cause electrons of the broad beam to impinge on thescanned areas when they are scanned and to periodically discontinue theflow of electrons oi the broad beam to scanned areas and thereby permitthe scanned areas to be restored to their initial potentials prior torescanning.

25. In cathode-ray tube signal reproducing apparatus for producing, on aluminescent screen, records of electric signals which persist after thesignals have passed, the combination which comprises a control screenhaving a secondary-electron emissive surface elemental areas of whichhave substantial resistance therebetween, means for producing a scanningelectron beam of elemental cross-section and a broad electron beam ofrelatively large cross-section, the electrons in the scanning beamhaving higher velocities than the electrons in the broad beam, aluminescent surface adapted to be rendered luminescent by electronsimpinging thereon and positioned to be impinged by electrons of saidbroad beam under the control of said emissive surface. means forperiodically scanning a selected area of said emissive surface with saidscanning beam in a plurality of lines in the presence of said broad beamelectrons, saidbroad beam substan- I release secondary electronstherefrom at a ratio greater than unity and do not produce substantialluminescence of said luminescent surface, whereas at said potentials thefaster electrons in the scanning beam liberate secondary electrons fromthe scanned elemental areas at a ratio greater than unity, means forcollecting the secondary electrons liberated by the scanning beam at apotential sufllciently high so that the potentials of the scannedelemental areas are raised to at least the level at which electrons inthebroad beam thereafter impinge thereon at increased velocitiessuflicient to maintain said scanned elemental areas at at least said'potential level by secondary electron emission therefrom at a ratiogreater than unity, whereby the raised potentials of the scannedelemental areas persist after the scanning beam has passed on so long aselectrons in the broad beam continue to impinge thereon, said raising ofthe potentials of the scanned elemental areas admitting electrons in thebroad beam to saidluminescent surface to produce a luminescent imagethereon, and means correlated with the deflection of the scanning .beamfor periodically deflecting sa d broad beam in a direction substantiallyperpendicular to the direction of the scanning lines at substantiallythe same velocity as that of the scanning beam in that direction tocause electrons of the broad beam to impinge on the scanned elementalareas when they are scanned and to cause said band free of broad beamelectrons to move across the selected area and thereby periodicallypermit the scanned elemental areas to be restored to their initialpotentials prior to rescanning.

26. In cathode-ray tube signal reproducing apparatus for producing, on aluminescent screen, records of electric signals which persist after thesignals have passed, the combination which comprises a control screenhaving a secondary-elec tron emissive surface elemental areas of whichhave substantial resistance therebetween, means for producing a scanningelectron beam of elemental cross-section and a broad electron beam ofrelative large cross-section, the electrons in.

i scanning elemental areas of said emissive surface with said scanningbeam inthe presence of said broad beam electrons, means for maintainingelemental areas of said emissive surface initially,

prior to the passing of the scanning beam thereover, at such lowpotentials that the slower electrons oi. the broad beam do not releasesecondary electrons therefrom at a ratio greater than unity and do notproduce substantial luminescence of said luminescent surface,'whereas atsaid potentials the faster electrons in the scanning beam liberatesecondary electrons from the scanned areas at a ratio greater thanunity, means for collecting the secondary electrons liberated by thescanning beam at a potential sumciently high so that 'the potentials ofthe scanned areas are raised to at least the level at which electrons inthe broad beam thereafter impinge thereon at increased velocitiessuificient to maintain said scanned areas at at least said potentiallevel by secondary electron emission therefrom at a ratio greater thanunity, whereby the raised potentials of the scanned areas persist afterthe scanning beam has passed on so long as electrons in said broad beamcontinue to impinge thereon, said raising of the potentials of thescanned areas admitting electrons in the bread beam to said luminescentsurface to produce a luminescent image thereon, and means for modulatingthe crosssectional area of the scanning beam at the control screen inaccordance with the signal to be reproduced.-

27. In cathode-ray tube signal reproducing apparatus for producing, on aluminescent screen, records of electric signals which persist after thesignals have passed, the combination which comprises a control screenhaving a secondary-electron emissive surface elemental areas of whichhave substantial resistance'therebetween, means for producing a scanningelectron beam of elemental cross-section and a broad electron beam ofrelatively large cross-section, the electrons in the scanning beamhaving higher velocities than the electrons in the broad beam, aluminescent surface adapted to be rendered luminescent by electronsimpinging thereon and positioned to be impinged by electrons of saidbroad beam under the control of said emissive surface, means fordeflecting said scanning beam to scan said emissive surface in linescomposed of a series of substantially discrete points, the scanning. oielemental areas oi said emissive surface being in the presence ofelectrons of said broad beam, means for maintaining elemental areasofsaid emissive surface initially, prior to the passing of the scanningbeam thereover, at such low potenials that the slower electrons of thebroad beam do not release secondary electrons therefrom at a ratiogreater than unity and do not produce substan- 2 tie] luminescence ofsaid luminescent surface,

whereas at said potenials the faster electrons in the scanning beamliberate secondary electrons from. the scanned areas at a ratio greaterthan unity, means for collecting the secondary electrons liberated bythe scanning beam at a potential sufiicientlyhigh so that the potentialsof the scanned areas are raised to at least the level at which electronsin the broad beam thereafter impinge thereon at increased velocitiessumcient to maintain said scanned areas at at least said potential levelby secondary electron emission therefrom at a ratio greater than unity,whereby the raised potentials of the scanned areas persist after thescanning'beam has passed on so long as electrons in said broad beamcontinue to impinge thereon, said raising of the potentials of thescanned areas admitting electrons in the broad beam to said luminescentsurface to produce a. luminescent image thereon, and means formodulating the scanning beam in accordance with the signal to bereproduced.

28. In cathode-ray tube reproducing apparatus for producing on aluminescent screen, records of electric signals which persist after thesignals have passed, the combination which comprises an electron gunadapted to produce a scanning electron beam of elemental cross-section,a second electron gun adapted to produce a relatively broad electronbeam, said electron guns having respective cathodes, means formaintaining the cathode of the first electron gun at a. lower potentialthan the cathode of the second electron gun, a control screen positionedin the path of said electron beams having a conductive member and asecondary-electron emissive surface exposed to said electron beams,elemental surface areas 01' said emissive surface having substantialresistance therebetween and being connected to the conductive memberthrough substantial resistance, a collecting anode positioned to collectsecondary electrons emitted from said emissive' surface, a luminescentscreen positioned and adapted to receive electrons of said broad beamunder the control of said control screen, means for deflecting saidscanning beam to scan said emissive surface in lines composed of aseries of substantially discrete points, the scanning of elemental areasor said emissive surface being in the presence of electrons of saidbroad beam, means for maintaining the potential of said conductivemember at a potential near that of the cathode of the second electrongun such that initially, ,prior to the passing of the scanning beam overelemental areas of the emissive surface, the broad beam electrons aresubstantially prevented from producing an image on said luminescentscreen and the potential difference between the elemental surface areasand the cathode of the second electron gun is less than required forelectrons in the broad beam to raise the'potentials of said areas bysecondary-electron emission therefrom at a ratio greater than unity, thepotential of the cathode of the first electron gun being selected withrespect to the potential oi the conductive member so that electrons inthe scanning beam liberate secondary electrons from scanned areas at aratio greater than unity, the potential oi the said collecting anodebeing maintained suiliciently high so that the potentials of the scannedareas are raised by the said liberation of secondary electrons therefromto at least the potential level at which electrons in the broad beamalso liberate secondary electrons in a ratio greater than unity andsumcient to balance the leakage current from the scanned areas to theconducting member, thereby maintaining the scanned areas at at leastsaid potential level so long as electrons in the broad beam continue toimpinge thereon, said potential level admitting electrons of said broadbeam to said luminescent screen to produce an image thereon, means formodulating the cross-sectional area of the scanning beam at the controlscreen in accordance with the signal to be reproduced, and means forperiodically discontinuing the flow of electrons in the broad beam tothe scanned areas to permit the charge on the scanned areas to leak awayto the conductive member and restore the scanned areas to substantiallytheir initial potentials at which the electrons in the broad beam aresubstantially prevented from producing an image on the luminescentscreen.

palms 0.4503.

